1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to an apparatus and method for cleaning an ion source chamber used in ion implantation equipment.
2. Discussion of Related Art
Ion implantation is a process used to dope impurity ions into a semiconductor substrate to obtain desired device characteristics. An ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. FIG. 1 is a block diagram of an ion implanter 100 including an ion source chamber 102. A power supply 101 supplies the required energy to source 102 which is configured to generate ions of a particular species. The generated ions are extracted from the source through a series of electrodes 104 and formed into a beam 95 which passes through a mass analyzer magnet 106. The mass analyzer is configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer for maximum transmission through the mass resolving slit 107. Ions of the desired species pass from mass slit 107 through deceleration stage 108 to corrector magnet 110. Corrector magnet 110 is energized to deflect ion beamlets in accordance with the strength and direction of the applied magnetic field to provide a ribbon beam targeted toward a work piece or substrate positioned on support (e.g. platen) 114. In some embodiments, a second deceleration stage 112 may be disposed between corrector magnet 110 and support 114. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy.
The ion source chamber 102 typically includes a heated filament which ionizes a feed gas introduced into the chamber to form charged ions and electrons (plasma). The heating element may be, for example, a Bernas source filament, an indirectly heated cathode (IHC) assembly or other thermal electron source. Different feed gases are supplied to the ion source chamber to obtain ion beams having particular dopant characteristics. For example, the introduction of H2, BF3 and AsH3 at relatively high chamber temperatures are broken down into mono-atoms having high implant energies. High implant energies are usually associated with values greater than 20 keV. For low-energy ion implantation, heavier charged molecules such as decaborane, carborane, etc., are introduced into the source chamber at a lower chamber temperature which preserves the molecular structure of the ionized molecules having lower implant energies. Low implant energies typically have values below 20 keV. When a particular feed gas is supplied to source chamber 102 to produce a desired ion species, additional unwanted species, either ions or neutrals, may also be produced. These unwanted species typically have low vapor pressure and may condense and adhere to the interior surfaces of the source chamber. For example, when phosphine (PH3) is fed into the source chamber, phosphorous (P) deposits may form on the chamber walls. When heavy molecules such as decaborane and carborane are fed into the source chamber, unwanted deposits on the source chamber walls and electrodes is more prevalent. These solid deposits may change the electrical characteristics (voltage instability) of the chamber walls and possibly interfere with the ion source aperture from which the ions are extracted, thereby causing unstable source operation and non-uniform beam extraction.
One method used to clean the ion source chamber includes the introduction of a cleaning gas such as, for example nitrogen triflouride (NF3), dichlorine (Cl2), sulfur hexaflouride (SF6), etc., which etches away the unwanted deposited material via plasma-enhanced chemical reaction. These gases are supplied to the ion source chamber at typically high flow rates. However, once the cleaning process begins a determination must be made when to stop the supply of cleaning gas to the source chamber. Currently, this endpoint determination requires equipment in addition to the ion implanter. For example, a Residual Gas Analyzer (RGA) or Optical Emission Spectroscopy may be employed for endpoint detection of source chamber cleaning. An RGA includes an ionizer, a mass analyzer and an ion detector. The RGA analyzes the neutrals outside the beam line and outputs a spectrum that shows the relative intensities of the various species present in the gas. The ions from the gas are distinguished by their masses by the analyzer of the RGA. Once the RGA determines that the species present in the gas corresponds to the material which comprises the ion source chamber the cleaning process is stopped. However, using RGAs for endpoint detection requires an additional piece of equipment other than the implanter itself. In addition, RGAs do not detect the ions created in and extracted from the ion source, but detect neutral gas atoms and molecules originating from all regions in the vacuum system. Thus, there is a need for endpoint detection of an ion source cleaning process utilizing existing equipment tools.